1. Field of the Invention
The present invention relates to an image formation apparatus including a hot-roll type fixing device and a method for determining malfunction of a temperature sensor in the same, and more particularly to determination as to whether malfunction occurs or not in a non-contact type temperature sensor.
2. Description of the Related Art
In image formation apparatuses such as photocopiers or printers, after a toner image is transferred onto a sheet, heat and pressure are applied to this toner image using a heat roller to fix the image on the sheet. In order to perform such a fixing process more appropriately, temperature management of the heat roller is important. Conventionally, in order to perform such heat roller temperature management, a variety of methods for measuring the surface temperature of the heat roller have been proposed.
Conventionally, since the surface of the heat roller is covered with a hard material, a contact type temperature sensor has been used which comes into contact with the roller surface to detect the surface temperature. In such a contact-type temperature sensor, since the temperature detection is performed directly, detection errors resulting from attachment of dust, toner, paper dust and the like in the apparatus are relatively few.
By contrast, recently, the surface of the heat roller is covered with a soft material and the surface of the heat roller receives a scratch by being contact with the temperature sensor, so that the contact type temperature sensor as described above cannot be used. Therefore, a non-contact type temperature sensor is proposed which can detect the surface temperature without coming into contact with the heat roller.
For example, Japanese Laid-Open Patent Publication No. 2004-151471 discloses an image formation apparatus including a roller heat sensing sensor for sensing heat radiating from a heat roller and an ambient temperature sensing sensor for sensing the ambient temperature of the roller heat sensing sensor.
When such a non-contact type temperature sensor is used, means for preventing detection errors or deterioration of detection accuracy due to attachment of dust, toner, paper dust and the like is required. For example, Japanese Laid-Open Patent Publication No. 2000-259033 discloses an image formation apparatus including non-contact type surface temperature detection means provided in non-contact with a fixing roller surface for detecting a surface temperature of a fixing roller and contact type surface temperature detection means provided to be able to contact with the fixing roller for detecting the surface temperature of the fixing roller. According to this image formation apparatus, a detection state of the non-contact type surface temperature detection means is determined based on a fixing roller surface temperature detection signal based on the detection result of the non-contact type surface temperature detection means and a fixing roller surface temperature detection signal based on the detection result of the contact-type surface temperature detection means. This image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-259033 requires a mechanism that allows the contact-type surface temperature detection means to come into abutment with or go away from the fixing roller and is disadvantageously increased in size and complicated as a whole.
By contrast, Japanese Laid-Open Patent Publication No. 2000-259035 discloses an image formation apparatus capable of sensing a malfunction in a sensor without using a contact type temperature sensor. This image formation apparatus includes infrared radiation detection means for detecting infrared radiation to convert the amount thereof into an electrical signal, temperature compensation means for performing temperature compensation of the infrared radiation detection means, and malfunction sensing means for sensing a malfunction of the temperature compensation means by observing a signal based on the output of the temperature compensation means with respect to a signal based on the output of the infrared radiation detection means.
Furthermore, Japanese Laid-Open Patent Publication No. 2006-047411 discloses an image formation apparatus provided with a non-contact type temperature sensor for sensing a temperature of a heat roller, wherein a correction temperature for correcting a temperature sensed by the non-contact type temperature sensor is determined by comparing a sensed temperature increase time required for a temperature sensed by the non-contact type temperature sensor to attain from a first set temperature to a second set temperature with a reference temperature increase time, which serves as a reference, required for the surface temperature of the heat roller to attain from the first set temperature to the second set temperature.
However, in the image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-259035, a change of the signal based on the output of the temperature compensation means with respect to a change of the signal based on the output of the infrared radiation detection means per a prescribed time is observed in order to sense malfunction of the temperature compensation means. Therefore, if dust, toner, paper dust or the like attaches to the temperature compensation means (e.g. thermistor) causing the entire offset in the output thereof, malfunctions cannot be detected.
Furthermore, in the image formation apparatus disclosed in Japanese Laid-Open Patent Publication No. 2006-047411, since the correction temperature is determined based on the time required for the temperature sensed by the non-contact type temperature sensor to yield a prescribed temperature increase, proper correction cannot be performed if the entire offset is caused in the output of the non-contact type sensor, as described above. Moreover, correction is performed based on a temperature change between two points of the sensed temperature, so that such correction that reflects the behavior of the sensed temperature during the course cannot be performed.